The subject matter disclosed herein relates generally to combined-cycle (CC) power generation systems.
Increased cyclic duty requirements, higher fuel costs, competitive deregulated energy markets, and stringent environmental regulations are resulting in a demand for faster sequences from CC power generation system operations. Furthermore, power generation system owners manage different startup objectives depending on local environmental regulations, energy dispatch requirements, and current fuel and energy prices. A typical startup objective is the reduction of startup time. However, the power generation system operator may need to minimize emissions, fuel costs, or net heat rate. Additional flexibility is also useful due to some startup procedures occurring over several hours and the potential for load, market, or power system conditions to change during that time period.
Commonly assigned US20070055392, filed 6 Sep. 2005, which is herein incorporated by reference in its entirety, describes a system and method for model predictive control of a power generation system. The control system includes a model for a number of power generation system components, and the model is adapted to predict behavior of the power generation system components. The system also includes a controller that receives inputs corresponding to operating parameters of the power generation system components and improves performance criteria of the power generation system according to the model.
Some combined-cycle power systems include at least two gas turbine engines (GTs). Each GT is coupled with a heat recovery steam generator (HRSG). Exhaust gases from each GT are channeled into the corresponding HRSG to generate steam for use in other power generation system processes such as driving a steam turbine assembly (ST).
Power generation systems with multiple GTs have more flexibility when selecting a startup process than systems with only one GT. The choice of GT loads at which each HRSG is connected to the ST (connecting point) and the choice of the order in which the GTs are connected to the ST (startup sequence) may be used to achieve improved startup performance. However, the best choice from such options is not always clear. Additionally, any startup sequence must be managed while factoring in power generation system and component constraints. Several examples of constraints include maximum stresses in the ST rotor, maximum differential expansion or minimum clearances between adjacent rotating and stationary parts, and maximum metal and steam temperatures.
In many combined-cycle power system startup processes, a first GT to roll off from turning gear and fire is designated as a lead GT, and a second GT is designated as a lag gas turbine. Blending the lag GT steam into the ST sometimes produces increased temperature gradients within various ST components. Depending on the magnitude of the temperature gradients, thermal stresses may be induced or increased within the ST. If the GT loading rates are very high, large thermal gradients may be developed in the ST, leading to high stresses and uneven thermal expansion that could result in rubs. Conversely, slow GT loading rates ensure a safe operation but increase fuel costs and reduce power generation system availability.
One challenge in generating optimal power plant control actions during transient operation is dissimilar time scales of dynamics of different components. For example, during a startup process, changes in the GT effectors (like fuel valve openings and inlet air guide vane angles) modify the GT state in a matter of seconds, while the resulting effects on stresses and clearances manifest themselves after relatively longer delays, typically in the range of ten minutes to thirty minutes. If the controller does not have the capacity of accurately predicting these longer term or “future” stresses or clearances, then the applied GT loads are typically conservative and include very low load rates to prevent thermally over-stressing the ST. Another transient operation consists in blending the steam generated in the lag HRSG into the ST. To prevent overstressing of the ST, at least some known combined-cycle power systems manually blend the steam generated within the lag HRSG over an extended period of time. However, slowly blending the lag steam into the ST may result in unnecessary delays to complete the blending. Sometimes, the transient operation constitutes an event that is manually controlled, and the operator has to decide when to trigger the event while ensuring future constraint (or boundary) compliance. Control guidance to trigger the event is typically conservative in the sense that unnecessarily long delays may be introduced before the event is allowed. Commonly assigned application Ser. No. 12/040,296, filed 29 Feb. 2008, describes a method for determining timing of the introduction of steam from the second HRSG to reduce this efficiency loss.
It would be useful to further improve operation of combined cycle power generation systems to improve startup conditions of the power generation systems and its components.